Andreas Westhoff

Flow Structure Formation of Turbulent Mixed Convection in a Closed Rectangular Cavity

An experimental investigation of flow structure formation in turbulent mixed convection in a closed rectangular cavity with an aspect ratio of 1:1:5 and air as working fluid is presented. Mixed convection at Re = 1.1·104 and Ra = 3.0·108 is studied under well-defined conditions by combination of forced and thermal convection. The resulting flow structures strongly depend on the ratio of inertia and buoyancy forces. A 2D mean wind, which can be approximated by a solid body rotation, is found at pure forced convection. With increasing Archimedes number (Ar), realized by a temperature gradient between bottom and ceiling of the convection cell, this structure becomes instable. Leading to four convection rolls for Ar = 3.4, which are oriented in longitudinal direction of the cell, are observed.

Experimental Investigation of Flow Structure Formation in a Heated Duct Flow

The pattern formation of a Poiseuille flow in a rectangular duct, which is heated from below, is studied experimentally for the Reynolds number Re = 200 within the Rayleigh number range 4 ×108 ≤ Ra ≤ 7 ×108 . The channel has an aspect ratio of Γ yz = width : height = 25 : 2. As working fluid, water (Pr ≈ 6) is used. In order to study the influence of buoyancy on the forced flow, velocity fields are measured plane parallel to the heated bottom plate by means of Particle Image Velocimetry (PIV). Furthermore, the measured data is analyzed concerning the flow structure formation as a function of the temperature difference between the inflow and the bottom of the duct. In this connection, two different mechanisms can be distinguished, which cause the transition from a laminar to a turbulent flow: a successively progressing transition on the one hand and, above a certain Archimedes number Ar, an abrupt transition.

Oscillations of Large-Scale Structures in turbulent Mixed Convection in a rectangular enclosure

The term mixed convection (MC) is used to describe the process of heat transfer in fluids where forced convection (FC) and thermal convection (TC) coexist. Mixed convection is an often occurring flow condition e.g. in the oceans, atmosphere, indoor climatisation or industrial processes and applications [1]. In many flow situations convection is the prevalent transport mechanism of heat whereas the heat transfer strongly depends on the dynamics of the largescale structures. In this study we investigate the formation of large-scale circulation (LSC) in mixed convection and the influence of the dynamics of the LSC, also known as mean wind, on the heat transfer.

Large-scale Coherent Structures in Turbulent Mixed Convective Air Flow

We report on an experimental study of mixed convective air flow in a cuboidal container with an inlet and outlet slot at \(\mathcal {A} r = 3.3\), \(\mathcal {R} e= 1.0 \times 10^4\), \(\mathcal {R} a= 2.4 \times 10^8\) at a fluid pressure of \(P = 11.6\,{\mathsf {bar}}\). Particle image velocimetry were performed with the objective to identify the dynamics of the large-scale circulations. In addition, we applied temperature measurements to determine the heat transport. Due to the unsteady nature of mixed convection in the present configuration, the velocity vector fields were subjected to a proper orthogonal decomposition in order to extract the predominant coherent structures. The analysis uncovers two structures consisting of three and four thermally induced large-scale circulations, which are arranged in longitudinal direction. Further, we found that the dynamics and the topology of the convection rolls strongly influences the heat transport between the container‘s inlet and outlet.

A Method for Optical Measurements of Height Distributions in Transparent Media

As light passes the interface between two media with different densities, the light will be refracted which leads to an optical beam displacement. In this paper we present a simple optical and non-invasive measurement technique and a reconstructing algorithm needed to determine the height distribution of a transparent medium. The local height of one medium is reconstructed using Snell’s law after measuring the beam displacement. Thereto, the optical displacement on a two dimensional background plane is obtained by using background oriented schlieren and spatial cross-correlation with reference imaging. Moreover, to rebuilt the three dimensional height distribution from the image plane, a camera calibration is required. As long as the interface between the two media is plan-parallel to the background plane, the presented method provides a unique solution with only one camera. In this paper we show that this simple reconstruction algorithm can also be used for reconstructing weakly curved interfaces as well. We also present results of measurement of water droplets on a planar surface, using this algorithm. Furthermore, we discuss the errors, which result from the non plan-parallelity of the interface and show that for strongly curved interfaces the method must be advanced by using spatial displacement gradients or multiple cameras, which allows to reconstruct curvy interfaces more accurately.