Characteristic scales and zonostrophy index estimated from experimental data.
Abstract
In this contribution, we present a set of procedures developed to identify fluid flow structures and characterize their space-time evolution in time-dependent flows. In particular, we consider two different contests of importance in applied fluid mechanics: 1) large-scale almost 2D atmospheric and oceanic flows and 2) flow inside the left ventricle in the human blood circulation. For both cases, we designed an ad hoc experimental model to reproduce and deeply investigate the considered phenomena. We will focus on the post-processing of high-resolution velocity data sets obtained via laboratory experiments by measuring the flow field using a technique based on image analysis. We show how the proposed methodologies represent a valid tool suitable for extracting the main patterns and quantify fluid transport in complex flows from both Eulerian and Lagrangian perspectives.
Keywords
- pattern identification
- laboratory experiments
- image analysis
- rotating turbulence
- flow in the left ventricle
1. Introduction
In most of the fluid flows of interest in nature and technology (i.e., geophysical flows, blood flow in the human circulation as well as flows in turbomachinery and around vehicles) the presence of turbulence in normally observed; therefore, their reproducibility and repeatability have always represented a crucial issue. In this regard, it is widely recognized that laboratory experiments represent a valid tool for the simulation and investigation of complex fluid flows under controlled conditions. With the improvement of measuring techniques, the possibility of acquiring huge high-resolution data sets in space and time is continually increasing. It is then fundamental to consider procedures suitable for a proper analysis of these data aimed at the definition and the characterization of the main flow pattern and of their evolution. In this contribution, we consider two examples of different contests of importance in applied fluid mechanics: 1) β-plane turbulence in the framework of large-scale almost 2D atmospheric and oceanic flows and 2) effect of artificial valves on the flow in the left ventricle in the framework of an in vitro model of human blood circulation. In both cases, the complexity of the flow arises from the embedded non-linear phenomena i.e., interaction of structures at different scales, the interplay between vortices waves and turbulence, anisotropy in the energy transfers, and in transport phenomena. Due to chaotic advection, the Lagrangian motion of passive particles can be very complex even in regular, i.e., non-turbulent, flow fields [1] as in the situations here discussed in which we considered almost 2D and time-periodic velocity fields. The chapter is organized as follows. In Section 2, we describe the case studies and the considered experimental apparatus. Theory, its application to the experiments, and the different post-processing methodology are described in Section 3, Section 4 contains some results. We discuss and give our conclusions in Section 5.
2. Material and methods
We provide below the description of the experimental models designed to reproduce: 1) turbulent flows affected by a
2.1 Rotating turbulent flows with a β -effect
In rotating turbulent flows, the latitudinal variation of the Coriolis parameter, the so-called
In this contest, in addition to the characteristic scales of 2D turbulence [7] associated with the small-scale forcing
To deeply investigate these features, we carried out several experimental campaigns in a rotating tank facility available at the Hydraulics Laboratory of the Sapienza University of Rome. As reported in previous papers [9, 10], the experimental setup consists of a square tank 1 m in diameter placed on a rotating table whose imposed rotation is counter-clockwise in order to emulate flows in the Northern hemisphere of a planet. To simulate the dynamics associated with the latitudinal variation of the Coriolis parameter in the Polar Regions, we consider the effects induced by the parabolic shape assumed by the free surface of a rotating fluid. In fact, it is represented by a quadratic variation in
In particular, a local Cartesian frame of reference at the midlatitude of the tank (
We perform a set of runs in which the magnets are located along an arc of latitude in the range 180° <
2.2 Flow in the left ventricle in the human blood circulation
The overall functionality of the heart pump is strongly related to the intraventricular flow features. Complexity in the ventricular flow is mainly due to fluid-wall interactions and turbulence onset in correspondence of the boundaries, three-dimensionality, and asymmetry in the pattern development. Here, the focus is on the investigation of the flow in the left ventricle (LV) during a cardiac cycle: it consists of an intense jet forming downstream of the mitral valve and in the development of the related coherent structures i.e., a vortex ring, which grows up during the systole, impinges on the ventricle walls and vanishes almost completely during the systole. A deeper analysis of the flow pattern evolution has shown on one hand that the observed flow structure appears to be favorable to ejection through the aortic valve during the systole [16] and on the other hand the mutual relationships between the formation and development of coherent structures in the LV and its functionality. Actually, one of the main reasons for the deviation from physiological conditions is represented by the replacement of the mitral valve with a prosthetic one, which obviously causes deep modifications in the hemodynamics and, consequently, in the associated flow pattern [17, 18, 19].
We reproduce in the laboratory the ventricular flow by means of a pulse duplicator widely described in previous papers [19, 20, 21], below we summarize its working principle. A flexible, transparent sack made of silicone rubber (wall thickness ∼ 0.7 mm) simulates the LV allowing at the same time for the optical access. The model ventricle is fixed on a circular plate, 56 mm in diameter, and connected to a constant-head tank by means of two Plexiglas conduits. Along the outlet (aortic) conduit a check valve was mounted, whereas different types of valves were placed on the inlet (mitral) orifice.
We consider three different scenarios: a) the inlet was designed in order to obtain a uniform velocity profile at the orifice mimicking physiological conditions, b) a monoleaflet (Bjork–Shiley monostrut) in mitral position 3) a bileaflet bicarbon prosthetic valve in mitral position; both valves were 31 mm in nominal diameter. The model of the LV was placed in a rectangular tank with Plexiglas (transparent) walls; its volume changed according to the motion of the piston, placed on the side of the tank. The piston was driven by a linear motor, controlled by means of a speed-feedback servo-control. The motion assigned to the linear motor was tuned to reproduce the volume change by clinical data acquired in vivo by echo-cardiography on a healthy subject [20].
2.3 Measuring technique
Two-dimensional velocity fields are measured by means of an image analysis technique called Feature Tracking, FT [22, 23]. The measurement chain can be summarized in the following steps: 1) identification of a proper measurement plane in the fluid domain; 2) seeding of the working fluid with a passive tracer; 3) illumination of the measurement plane previously identified; 4) image acquisition; 5) image pre-processing of the acquired images; 6) particle detection and temporal tracking to isolate particles and track them in consecutive frames; 7) data post-processing to obtain the relevant flow parameters. Obviously, flow images are acquired at a certain space–time resolution, depending on the characteristic time and length scales of the investigated phenomena, the details for each apparatus are provided in the corresponding subsection.
Pre-processing includes the sequence of operations carried out to improve the quality of acquired images for the subsequent core of the processing phase. Basically, the procedure implies the background removal as well as the removal of parts of the image which are not significant for the flow analysis as for instance regions close to the boundaries. In fact, the glares due to the interaction between the lighting system and the domain walls may affect the processing algorithm.
FT is a multi-frame algorithm based on the assumption of image light intensity conservation in space and time between two successive frames and in the neighborhood of the seeding particles; this assumption holds for small time intervals. The algorithm essentially considers measures of correlation windows between successive frames and evaluates displacements by considering the best correspondence (in terms of a defined matching measure) of selected interrogation windows between subsequent images. Sparse velocity vectors are then obtained by dividing the displacement by the time interval between two frames; FT then provides a Lagrangian description of the velocity field. These sparse data can be interpolated on a regular grid through a resampling procedure allowing for the reconstruction of the instantaneous and time-averaged Eulerian velocity fields as well. The advantage of having at the same time both the Lagrangian and the Eulerian description of the flow is evident; in addition, if compared to other tracking algorithms, FT is not constrained by low seeding density, so it provides accurate displacement vectors even when the number of tracer particles within each image is very large [22].
3. Data analysis
3.1 Traveling waves and eddies
As mentioned before, in these jet flows waves and eddies co-exist; to highlight the propagation of the traveling structures in the physical space, we consider both a measure based on Hovmöller diagrams and the theoretical phase speed of the Rossby wave.
As for the former, we map the time evolution of the stream function
then the net speed of the propagating structures is evaluated by subtracting the mean zonal velocity from
where
As for the theoretical speed, we have shown in [14] how to derive the dispersion relation of a linear Rossby wave in polar coordinates; here, we reported the final expression:
being R the radius of the device (in this case the radius of the circle inscribed in the square tank),
In oceanography, one of the most popular methods used to detect coherent long-lived coherent structures, such as mesoscale eddies, is based on the estimation of the Okubo-Weiss parameter [24, 25]. This quantity describes the relative dominance of deformation with respect to rotation of the flow and it is defined as:
where
3.2 Finite-time Lyapunov exponents and Lagrangian coherent structures
Finite-Time Lyapunov Exponents (FTLE) represents a powerful tool suitable to track coherent structures and to unveil their connections to energetic and mixing processes, in fact, it has been used extensively in different contexts, including biological and geophysical flows [28, 29]. Basically, the FTLE measure the maximum linearized growth rate of the distance among initially adjacent particles tracked over a finite integration time. In brief, the computation of FTLE follows from the definition of the flow map
mapping a material point x(t) at time t to its position at t +
Since the maximum stretching occurs when the initial separation is aligned with the maximum eigenvalue of Δ, the FTLE is defined as:
Where
In addition, Lagrangian Coherent Structures (LCS) can be inferred from FTLE, [31]. LCS analysis represents a very powerful tool in cardiovascular fluid dynamics [32]; allowing for the identification of stagnant fluid areas, which are associated with an increased risk of thrombus as well as with blood cell damage. In addition, it helps to discern the regions directly affected by the vortices within the fluid domain and, possibly, their, modifications related to pathologies. FTLE investigation was successfully applied to the analysis of data sets obtained from both numerical simulations [27] and in vitro study [33] of a mechanical heart valve, as well as for the in vitro investigation of coherent structures educed from two-dimensional velocity fields in a LV model [21]. Recently, FTLE is also being used in the analysis of data sets collected in vivo [34, 35] and have been recognized as one of the main methods for the analysis of Lagrangian transport in blood flows [36, 37].
4. Results and discussion
4.1 Rotating flows affected by a β-effect
Before running each experiment, the fluid surface is seeded with styrene particles (mean size
4.1.1 Waves and eddies propagation
In Figures 1 and 2 we plot the instantaneous and time-averaged flow fields obtained in one run (I = 4A) of the experiments WW and EW; the plots are shown hereafter refer to experiments performed using the same forcing amount. Figure 1 clearly shows a meandering jet squeezed between westward propagating eddies in the instantaneous flow field; on the contrary, the averaged field reveals strong alternating zonal jets and no eddies. These experimental features resemble ocean observations that highlight numerous westward propagating eddies on short time scales [12]. At the difference, the eastward jet is not associated with eddy shedding and traveling structures and the instantaneous and averaged flow appear to be rather similar (Figure 2).
To characterize the traveling structure observed in the WW case, we map the velocity stream function
As discussed in Section 3.1, by measuring the slope of the lines of the same color, we were able to estimate experimentally the speed of the propagating structures relative to the zonal flow with Eq. (2); we then calculate the theoretical speed using Eq. (3) and compare the obtained values. The comparison shows that the relative error, i.e. the ratio between the measured and the expected speed, is minimum in correspondence of
In order to compare our method to evaluate the eddies propagation speed with a method widely used in the applications we also applied an OW-based method to our experimental data sets. At first, we evaluated OW parameter through Eq. (4) at each time instant. Then, using a threshold of OW0 = 0.5
Finally, once identified the coherent vortices, we detected the center of each structure and tracked them in the considered time interval. We found values of the propagating speed close to the ones found through the Hovmöller diagrams. We conclude that waves and propagating eddies coexist in the zonal pattern and confirm their duality nature [14]. The application of the same procedure of analysis overall the EW experiments is actually in progress [38].
4.1.2 Characteristic scales and flow regime
The estimation of flow characteristic length scales is crucial to identify the flow regime in rotating turbulent flows with a
Run | |||
---|---|---|---|
1.41 | 2.20 | 1.55 | |
1.09 | 1.75 | 1.60 | |
0.94 | 1.16 | 1.70 | |
1.19 | 1.90 | 1.59 | |
0.85 | 1.45 | 1.70 | |
0.74 | 1.29 | 1.75 |
According to the classification provided in [8] we conclude that all our experiments reproduced flows in a transitional regime.
4.2 Flow patterns in the left ventricle downstream of prosthetic valves
To perform flow measurements in the LV, the vertical symmetry plane aligned with the mitral and aortic valve axes is illuminated by a 12 W, infrared laser. The working fluid inside the ventricle (distilled water) is seeded with neutrally buoyant particles (
For the dynamic similarity, we consider the Reynolds number
We use the public domain code NEWMAN [39] to compute the FTLEs from the planar velocity dataset above described, for the details see [40]. We remark that FTLE fields are computed from 2D measurements even if it is well known that the observed phenomenon is 3D; indeed, as the measurement plane is a plane of symmetry the assumption of two-dimensionality is quite acceptable.
Figure 6 shows backward FTLE at the end of the E-wave for the three simulated conditions. Backward FTLE ridges correspond to the front of the diastolic jet, sharply separating the fluid which just entered the ventricle from the receiving fluid.
The analysis of the FTLE patterns throughout the cardiac cycle (not shown here) highlights how in the physiological configuration the observed coherent structures appear to be optimal for the systolic function. Indeed, the modifications in the transmitral flow due to the presence of a prosthetic valve deeply impact on the interaction between the coherent structures generated during the first phase of the diastole and the incoming jet during the second diastolic phase. We observed that while the flow generated by a bileaflet valve preserves most of the beneficial features of the top hat inflow, downstream of a monoleafleat one the strong jet forming at the end of the diastole prevents the permanence of large coherent structures within the LV (Figure 7).
In order to complete the FTLE analysis, we reconstruct the trajectories of a number (O(104)) of synthetic fluid particles entering the ventricle through the mitral orifice during the LV filling by numerically integrating the experimental velocity fields; for each run, synthetic particles were released during each time step of the diastolic waves from the mitral orifice section and were subsequently tracked during the cardiac cycle. The aim was to further clarify the role of LCS by overlapping the particle positions on the FTLE maps, and to verify if and how LCS may act as pseudo-barriers for transport and mixing. An example is reported in Figure 8.
We finally compute the shear stress experienced by the particles along their trajectories in order to emphasize the differences among the simulated conditions and to clarify the possible implications on the hemodynamics. Results corresponding to the end of the A wave are shown in Figure 9.
The plots show that, in case (a) the stress magnitudes induced by the smoother flow pattern are lower than values measured in case (b) and (c). In fact, while in physiological conditions particles characterized by the highest shear are washed out by the systolic wave, in presence of prosthetic valves they tend to be advected towards regions of the LV not affected by the systolic ejection (see Figure 3).
5. Conclusions
In this work, we review a set of methodologies suitable for the characterization of time-periodic complex flows; in particular, here, the focus is on rotating flows affected by a
Acknowledgments
The authors would like to thank the Sapienza University of Rome (Research program SAPIEXCELLENCE SPC: 2021-1136-1451-173491), the European Union’s Horizon 2020 research and innovation program (Marie Sklodowska–Curie Grant Agreement No. 797012) and the Italian Ministry of Research (project PRIN 2017 A889FP).
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