Dr. Laila GuessousSuresh Putta, M.S. Student

Numerical Investigations of Pulsatile Flows

To develop a better understanding of the characteristics of pulsating flows and to investigate the effect of such pulsations on convective heat transfer. In particular:•Can turbulent convective heat transfer rates be enhanced or controlled through pulsation of fluid flow? The current literature disagrees about the effect of frequency and amplitude on the time-mean characteristics of pulsating flow and heat transfer•What are the effects of such pulsations on the underlying microstructure of turbulent flows and what are the subsequent implications for the modeling and correlation of unsteady turbulent heat transfer and flow?

Understanding the fundamental effects of an imposed frequency on the structure and heat transfer of a turbulent fluid flow would have far ranging implications in both industrial and biological applications. Unsteady or pulsatile turbulent flows occur in such problems as internal combustion engines and exhaust systems, turbomachinery, helicopter blades, cardiac blood flow, musical instruments, cooling systems, etc.

We are conducting Computational Fluid Dynamics (CFD) investigations of pulsating flows in a number of geometries (pipe and blunt flat plate). Pulsation is introduced by either oscillating the inlet pressure: P=Po(1+A cos t) , or the free-stream velocity: U=Uo(1+A sin t).

2-D numerical simulations of unsteady flow and heat transfer over a long rectangular plate at a mean Reynolds number of 1000 (based on plate plate thickness) are currently underway. In the absence of pulsation, this flow exhibits a natural vortex shedding frequency, n. In our simulations, we are perturbing the inlet free-stream velocity sinusoidally over a range of frequencies and examining the effects of these oscillations on the flow dynamics, wall heat transfer and temperature field. In particular, we are looking at the effect of harmonics of n. Sample plots showing the evolution of the streamlines for = 2n and A = 0.2 are shown below. The flow exhibits an unsteady vortex formation and shedding pattern. The size of the separation bubble and reattachment length varies with frequency. These changes in the flow coherent structures are expected to affect the rate of heat transfer at the wall.

Velocity profile at time,t=12.180,A=5,womersely number,W=5

0

0.002

0.004

0.006

0.008

0.01

0.012

-25 -20 -15 -10 -5 0 5

velocity

Velocity profile at time,t=12.480,A=5,womersely number,W=5

0

0.002

0.004

0.006

0.008

0.01

0.012

-5 0 5 10

velocity

velocity profile at time,t=12.580,A=5,womersely number,W=5

0

0.002

0.004

0.006

0.008

0.01

0.012

0 2 4 6 8 10 12 14

velocity

Velocity profile at time,T=12.760 ,A=5,womersely number,W=5

0

0.002

0.004

0.006

0.008

0.01

0.012

0 5 10 15 20

velocity

Velocity plots at time,t=11.5,A=5,Womersely number,W=5

0

0.002

0.004

0.006

0.008

0.01

0.012

-40 -30 -20 -10 0

velocity

Velocity profile at time,t=3.57,A=5,womersely number,W=10

0

0.002

0.004

0.006

0.008

0.01

0.012

0 5 10 15

velocity

rad

ial d

ista

nce

Time evolution of laminar oscillating flow in a pipe, 52DWo

Simulations are being performed for laminar oscillating and pulsating pressure gradients in a pipe. Results show that for low frequencies, the flow is able to adjust instantaneously to the variation in the imposed pressure gradient and exhibits a parabolic velocity profile. However, as the frequency increases, the velocity is unable to keep up with the temporal pressure changes and displays a phase lag. As can be seen in the following figures, high velocity gradients can be seen close to the wall and a plug-type flow is observed in the center of the pipe. These changes in velocity distribution are expected to affect the heat transfer at the wall.

1.Objective:

2. Motivation:

3. Approach:

4. Flow in a Pipe

5. Blunt Flat Plate: