Session: 10-04-01: Fluid Measurements and Instrumentation

Paper Number: 110625

110625 - Investigation of Frequency Coupling in a Restricted Pulsatile Flow 

The nonlinear nature of turbulence causes frequencies to interact triadically, forming new frequencies at the summation and difference of the coupled pairs. A comprehensive way of summarizing the nature of these triadic interactions is sought using a spectral measure of coherence between frequencies. By altering different characteristics of the flow, such as the boundary conditions or its velocity field, the measure of frequency coupling under these different conditions can be investigated.

Inspired by aortic valve stenosis, the nature of frequency coupling is investigated through acoustic measurements in a pulsatile flow with changing narrowing severity in the flow restriction. The pulsing frequency is set to 70 beats per minute (1.17 Hz), while the Reynolds number (Re = UD/v), based on the average bulk velocity per pulse U, hydraulic diameter of the opening D, and kinematic viscosity v, is varied in eight steps between 4500 and 9831 to mimic a range of human aortic valve conditions. A narrowing in the flow is produced through a 3D printed rigid valve model, based loosely on the shape of a tricuspid valve. The narrowing severity ranges from unrestricted through 82% restricted in eight increments based on a rating scale from the American Society of Echocardiography. The variation in Re and restriction percentage provides 64 distinct test conditions to allow an investigation into the influence of narrowing severity and flow dynamics on the acoustic spectrum. Utilizing both the power spectrum and the bicoherence, a quantitative measure of restriction severity based on the dominant frequencies present is sought.

Initial findings in the power spectrum of the acoustic signal indicate that the relative percentage of energy in the bands of 300 to 400 Hz rises with increasing narrowing percentage for a fixed Re. This trend also holds for fixed restriction percentages but increasing Re. These results are consistent with prior literature which indicated an increase in high frequency content in biological flows with stenosis. To refine the quantitative assessment, the bicoherence of each sound signal is calculated. The bicoherence reveals phase coupling between two or more interacting frequencies and their triadic sums, allowing insight into the prominent frequencies for the given flow parameters. For all restrictions and Re, the forcing frequency of 1.17 Hz and its harmonics are strongly coupled throughout the entire spectrum. As the entire flow is driven by this forcing frequency, this high level of coherence is expected. Early results looking at two individual restrictions indicate characteristic frequency bands of 240 – 270 Hz and 40 – 53 Hz for 25% and 69% restrictions, respectively, across all Re. Additionally, different sets of distinct frequency pairs appear in all 64 test conditions. A clear correlation between the frequency pairs and Re to reverse engineer the restriction percentage is still under investigation. Knowledge of this dependence would allow for an non-invasive measure of a restriction severity, which would find significant potential in medical diagnosis of aortic valve stenosis.

Presenting Author: Alexandra Barbosa Gonzalez Trinity College

Presenting Author Biography: Alexandra is a current undergraduate engineering major at Trinity College.

Authors:

Alexandra Barbosa Gonzalez Trinity College
Felix Goldmann Trinity College
Fadhil Ahmed Trinity College
Clayton Byers Trinity College Engineering