Experimental and Numerical Investigation of Oscillating Fluid Flow on Pipes

Abstract

Vibration in vehicles is due to two major reasons, the first due to road surface irregularities and the second due to engine piston firings and the crankshaft eccentricity. Primary suspensions, springs, and shock absorbers, reduce the vehicle vibrations due to road irregularities and secondary suspensions, engine mounts, control vehicle vibrations due to engine vibration.

A good vibration isolator must have properties to restrict the relative displacement of the engine as well as reduce the structure-borne transmitted force. Engine mounts are such vibration isolators that are employed in automobiles to mitigate the effects of engine vibration. The ideal engine mount should have low dynamic stiffness and damping characteristics for high-frequency low-amplitude vibrations and high dynamic stiffness and damping characteristics for low-frequency high-amplitude vibrations to minimise engine motions. Therefore, due to this dual dynamic stiffness behaviour and application in vehicles, hydraulic engine mounts, which is the focus of this thesis, are employed to mitigate the amplitude and frequency dependent vibrational effects of the engine.

There are two types of hydraulic engine mounts, double pumper, and single pumper hydraulic engine mounts. A single pumper hydraulic engine mount typically consists of a main rubber as upper compliance, upper and lower chambers filled with a fluid, a decoupler or no decoupler, an inertia track connecting the two fluid chambers, and a flexible rubber diaphragm as the lower compliance. The engine weight is supported by the primary rubber. The focus of this thesis is on single pumper hydraulic engine mounts (fluid mounts) with an inertia track but with no decoupler.

The inertia track is a tube with a specific length and cross-sectional area that connects the two fluid chambers. As the engine mount is subjected to vibration, the working fluid in the hydraulic engine mount will experience oscillatory fluid flow phenomenon. As a sinusoidal displacement due to vibration is applied across the chambers, the working fluid is forced to move through the narrow inertia track causing a pressure drop across the mount. To determine the efficiency and improve the performance of the hydraulic engine mounts, the prediction of this pressure drop is a key. Therefore, the magnitude of the pressure drop, which is a function of inertia track geometry, fluid viscosity, density and compressibility, and input frequency and amplitude, is analyzed in this thesis using MATLAB and ANSYS FLUENT.

In this thesis, prior publications focused on the design and experimental analysis of hydraulic engine mounts as well as publications focused on analytical and numerical simulation of pulsating fluid flow in pipes have been reviewed. Based on this literature review, the following research gaps have been identified in terms of numerical and analytical modelling of the oscillating fluid flow and experimental evaluation: that includes (i) no simple closed-form relations exist to calculate the flow losses in an inertia track of a given geometry for oscillatory fluid flows. (ii) only few computational fluid dynamic analyses have been conducted for oscillatory fluid flow through pipes, and (iii) no comparison has been made between numerical CFD simulation results and experimental results for oscillatory fluid flows in pipes except one.

In this thesis, considerable efforts have been dedicated to exploring the oscillatory fluid flow behavior of the working fluid within hydraulic engine mounts. A simplified physical model of a fluid mount has been devised, and corresponding numerical and mathematical models have been developed to analyze the impact of variables such as frequency and displacement on pressure loss across the inertia track. To validate these models, the pressure loss data from the inertia track obtained through both modelling approaches was compared. This methodology facilitates the development of correlations between input displacement, inertia track geometry, fluid density, viscosity, and pressure loss across the inertia track in hydraulic engine mounts, thereby enhancing their damping efficiency.

To further this research, a flow bench was designed, analyzed, and fabricated to simulate single pumper hydraulic engine mounts. The setup was equipped with purchased pressure gauges and transducers and included three different inertia tracks. The flow bench underwent extensive testing using several inertia track geometries, and input frequency and amplitude settings. Additionally, numerous CFD analyses paralleling these experiments were conducted using ANSYS Fluent to facilitate comparisons.

However, several challenges were encountered with the flow bench, including issues with air entrapment, leakage, and manufacturing defects, which hindered its effective utilization. Consequently, reliance was placed on previously collected experimental data on Double Pumper fluid mounts, which were documented by my advisor during his tenure in the industry. These CFD results have so far matched reasonably well with the experimental results for double pumper cases.

For single pumper hydraulic engine mounts, it is evident that the flow bench requires a thorough redesign and re-manufacturing to mitigate friction between the piston and chamber walls and to employ a proper metal spring. This redesign would ensure appropriate pressurization of the chambers to prevent fluid cavitation, thereby optimizing the functionality and efficiency of the mounts.

Date of Award	11 Jul 2024
Original language	American English
Supervisor	Nader Vahdati (Supervisor)