Control of Separated Flow Past Backward-Facing Step in Microchannel

Abstract

A key concern for micro device design is its power consumption. When such a device involves microflows, actively controlling the flow losses often reduces the power requirements. In the present study, a micro synthetic jet is proposed as a flow control device. The method used is an automated design optimization methodology coupled with computational fluid dynamics. Microflows in the Knudsen range of 10−3 to 10−1 are modeled using a Navier-Stokes solver but with slip velocity and temperature jump boundary conditions derived for micro-sized geometries. First, an uncontrolled flow past a backward facing step in a channel is computed. Then, a synthetic jet actuator is placed downstream of the step where the separation occurs. A large number of test cases have been analyzed. It has been observed that the reattachment point of the separated flow and the flow dissipation are quite sensitive to the location and the geometry of the synthetic jet as well as the parameters of the oscillating membrane. The best flow control, defined as the largest decrease in dissipation, is obtained when the actuator cavity width and the membrane oscillation amplitude are increased simultaneously.

Free-molecular and near-free-molecular gas flows over backward facing steps

“…Reviewing existing studies on rarefied gas flows over micro-steps (Beskok 2001;Baysal et al 2004;Xue et al 2005;Hsieh et al 2010;Bao & Lin 2011;Darbandi & Roohi 2011;Mahdavi et al 2014;Mahdavi & Roohi 2015;Gavasane et al 2018), the current work presents several new findings that are of fundamental and practical significance. In contrast to the common view, it is first demonstrated that step detachment may occur at free-molecular conditions, identified by a unique velocity profile.…”

“…In line with the above set of works, the channel flow over microscale steps has been investigated and recently reviewed (Kherbeet et al 2016). Focusing on the limit of small Knudsen (Kn) numbers, several studies have applied continuum-limit models to describe and monitor the detachment and reattachment phenomena in a slightly rarefied gas (Beskok 2001;Baysal, Erbas & Koklu 2004;Bao & Lin 2011). The majority of works have otherwise used the direct simulation Monte Carlo (DSMC) method to analyse the backward step flow at arbitrary Knudsen numbers.…”

Thermal Rarefied Gas Flow Investigations Through Micro-/Nano-Backward-Facing Step: Comparison of DSMC and CFD Subject to Hybrid Slip and Jump Boundary Conditions

“…Their simulation results provided numerical data for incompressible flows over a backward-facing step. Control of separated flow past a backward-facing step in a microchannel was studied by Baysal et al [7] based on the NSF equations with slip/jump boundary condition in the range 0.01 < Kn < 0.1. Xue and Chen [8] simulated micro-backward-facing step flows in both slip and transition flow regimes by the DSMC method.…”

Micro/nano thermal boundary layer equations with slip creep jump boundary conditions

“…where the second term of the tangential velocity boundary condition is the thermal creep which generates a slip velocity along non-isothermal solid boundaries, and k 2 is another constant related to the Knudsen number. These boundary conditions are sometimes referred to as the (first-order) Maxwell-Smoluchowski boundary conditions (Beskok & Karniadakis, 1999;Baysal et al, 2004), which in a way corrects the discrepancy that (1.1) is generally referred to as the Navier boundary condition (Matthews & Hill, 2006b). However, in this article and following the usage by Marques et al (2000), (1.3) will be referred to as the slip-creep-jump boundary conditions.…”