In-Situ Measurement of Wall Effect on the Hydrodynamic Force for a Solid Sphere in Normal Approach at Constant Velocity or Acceleration at Low Reynolds Number
Date Issued
2014
Date
2014
Author(s)
Chen, Hsien-Jen
Abstract
This thesis studies the hydrodynamic force on an immersed sphere approaching in normal direction toward a wall in constant velocity or constant acceleration motion by systematic experiments. The range of Reynolds number covers 0.21 to 3.33. A test sphere connected to a small capacity load cell was driven by a precision servo motor to achieve direct synchronized force measurement with sphere motion.
We firstly drove the sphere at constant velocities and accelerations near the center of a large tank for in-situ calibration and then drove the sphere at identical motion towards the wall. The downstream wall modifies flow boundary condition to result in hydrodynamic force different from the value found in infinite domain. The ratio between the forces with and without downstream wall quantifies the wall effect which is referred as the wall factor.
The wall factor measured for quasi-steady force falls between the predictions from two theoretical models and grows monotonically with diminishing gap. Besides of this calibrated quasi-steady drag, wall effect on amplifying the added mass force when the sphere in constant acceleration towards wall was modeled by potential flow theory. These two force components, with respective wall factors, are then subtracted from the total unsteady hydrodynamic force to extrapolate wall-modified history force. An empirical formula was then fitted for the wall factor for history force, which shows dependences on gap and acceleration.
Subjects
流體動態力即時量測
史托克阻力
流體歷史力
牆面修正係數
流體歷史力之牆面修正係數
Type
thesis
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