Mohammed Azharudeen Farook Deen

Animals exhibit an extraordinary capability for agile locomotion across various natural terrains. However, replicating this adaptability by developing controllers capable of traversing on soft and deformable terrains, especially at high speed, remains a challenge. Our study proposes an end-to-end learned approach for a fast and robust controller that aims to guide quadruped robots through a wide range of deformable substrates such as soft soil, loose gravel, rough, rocky, wet, muddy, etc. We use a simulation environment that can be parameterized to represent diverse types of terrain that closely mimic the natural world and train a policy network via model-free reinforcement learning. Our proposed approach includes the key components (1) implementing a multiscale granular media model for real-time simulation of terrain dynamics that combines an active zone resolved as distinct particles on top of a resting compliant terrain, (2) using a terrain adaptive curriculum for command velocities, and (3) an online adaptation module strategy that can implicitly identify the terrain properties for easy sim-to-real transfer to physical robot leveraged from prior works.

Recently, the phenomena of streaming suppression and relocation of inhomogeneous miscible fluids under acoustic fields were explained using the hypothesis on mean Eulerian pressure. In this work, we derive the expression for the acoustic body force without relying on any prior assumptions regarding the second-order Eulerian pressure. We present a theory of nonlinear acoustics for inhomogeneous fluids from first principles, which explains streaming suppression and acoustic relocation in both miscible and immiscible inhomogeneous fluids inside a microchannel. This theory predicts the relocation of higher impedance fluids to pressure nodes of the standing wave, which agrees with recent experiments.

In this paper, we demonstrate a heat transfer mechanism using ultrasonic standing waves. The basic idea behind the proposed heat transfer mechanism is the acoustic relocation phenomenon of inhomogeneous fluid due to acoustic body force. The acoustic body force depends upon the density gradient and the speed of the sound gradient of the inhomogeneous fluid. Heating a fluid creates an inhomogeneity in the physical properties of the fluid such as density, viscosity, and velocity of sound, etc. When this heated (inhomogeneous) fluid is subjected to ultrasonic standing waves, acoustic body force induces a fluid motion which is shown to be responsible for this heat transfer mechanism. Heat transfer enhancement is observed when a standing acoustic wave is passed perpendicular to the direction of heat transfer. Remarkably, it is found that acoustic forces can enhance heat transfer up to 2.5 times compared to natural convection and up to 11.2 times compared to pure conduction. Suppression of natural convection heat transfer is observed when the acoustic waves are passed parallel to the direction of heat transfer. In this case, acoustic forces could bring down the heat transfer by half or more than half from the natural convection. To characterize the heat transfer mechanism in the enhancement case, a modified Rayleigh number that can account for both acoustics and gravity effects is proposed. To this extent, we provide a clear understanding of how acoustic fields influence the fluid flow and heat transfer.

We present a technique for mixing the fluids in a microchannel using ultrasonic waves. Acoustic mixing is driven by the acoustic body force, which depends on the density gradient and speed of the sound gradient of the inhomogeneous fluid domain. In this work, mixing of fluids in a microchannel is achieved via an alternating multinode mixing method, which employs acoustic multinode standing waves of time-varying wavelengths at regular time intervals. The proposed technique is rapid, efficient, and found to enhance the mixing of fluids significantly. It is shown that the mixing time due to acoustic mixing (2–3 s) is reduced by two orders of magnitude compared to the mixing time only due to diffusion (400 s). Furthermore, we investigate the effects of the acoustic mixing on different fluid flow configurations and sound wave propagation directions as they have a direct influence on mixing time and have rarely been addressed previously. Remarkably, it is found that mixing performance is strongly dependent on the direction of the acoustic wave propagation. The acoustic field propagated parallel to the fluid-fluid interface mixes fluids rapidly (2–3 s) as compared to the acoustic field propagated perpendicular to the fluid-fluid interface (40 s).