1
Advanced Rover Systems
Presents
The Venturi-Vac
Venturi-based Vacuum Retrieval of Topsoil on Mars
March 23rd, 2001
Colorado School of Mines
Aaron Shock Kim Fleming
Juliana Drummond Sean Gilpin
Table of Contents
Executive Summary / 2Introduction / 3
History and Development
Original Design / 3
Venturi-Vac / 4
Pre-Development Concept Testing / 4
Experimental Testing / 5
Analysis of the Venturi Concept / 6
Technical Information / 6
Terminal Velocity / 7
Earth Calculations / 8
Mars Calculations / 9
Bernoulli Equation Analysis
Determination of Initial Velocity / 11
Experimental Findings / 14
Venturi Subsystem Analyses
Front-End Atmospheric Intake / 14
Venturi Chamber / 15
Collection System / 16
Future Project Goals / 18
Project Outreach / 19
Appendix A – References Cited
/ 20Executive Summary
The minimal availability of water on Mars presents a variety of problems for future space travel to, and exploration of the red planet. Presently, scientists predict that water is available for extraction from the finer particulate matter that makes up the top layer of the Martian lithosphere. Therefore, NASA and its subsidiaries have begun seeking possible answers to the problem of how to excavate Martian soil without human assistance, for the purpose of extracting water.
In September of 1999, Dr. Michael Duke of the Lunar and Planetary Institute presented the EPICS Department of Colorado School of Mines with a set of design specifications for an autonomously operable vehicle capable of excavating Martian surface soil. Most importantly, this problem statement memo specified that the vehicle must
- Excavate Martian soil to a maximum depth of 20 centimeters,
- Have a mass not to exceed 20 kilograms,
- Operate autonomously for up to 500 days with only one communication transmission per day from the mission control center, and
- Deliver the excavated soil to a reactor system
Therefore, beginning in September of 1999, the design partners of Advanced Rover Systems examined several possible solutions to this design problem. By December of 1999, we had finished a vehicle design that would effectively accommodate all of the necessary specifications. Advanced Rover Systems was subsequently given the opportunity to continue the development of our design by building a test-worthy prototype. But as we set forth to refine our original design, we derived a more efficient solution to the problem of excavating soil on Mars.
Subsequently, we investigated the possibility of retrieving surface soil by way of less mechanically intensive systems. Through basic experimentation, we discovered that we could lift soil off the ground in a manner similar to the way a traditional vacuum works. The vacuum based soil excavation system we have developed – the Venturi-Vac – incorporates minimal motorized parts, posing little risk of mechanical failure and requiring less power than our original design. As detailed in the following report, the Venturi-Vac is a highly effective soil excavation system
All written materials and drawings contained herein are the sole property of Advanced Rover Systems, with design patents pending.
Introduction
This project began as an EPICS151 design team assignment during the fall semester of 1999, derived from the project specifications originally presented by Dr. Michael Duke of the Lunar and Planetary Institute. These specifications called for the design of a soil excavation unit to operate autonomously on the surface of Mars. The unit needs to be functional for 500 days, and could only receive instructional transmissions from Earth once per day.
Through the course of an academic semester, our design team examined several options for the complete accommodation of these project specifications. When presented with the opportunity to refine our design and build a prototype for testing purposes, we discovered that we could accomplish the same tasks with a much less intricate soil retrieval system. Hence was born the Venturi-Vac, a vacuum-based soil lifting system that requires far fewer motorized parts and mechanical systems. Advanced Rover Systems believes that the Venturi-Vac will meet the project specifications and accomplish the given task of soil excavation more efficiently than more mechanically intensive designs.
While refining our ideas on the best possible way to implement a prototype of our design, we discovered several constraining factors that have altered our original plan for this phase of the project. Originally, we planned on fully implementing an autonomous vehicle capable of excavating soil under its own control. This version of the project scope included a functional control processor, suspension and drive train systems, as well as navigation and solar power systems. However, we opted to refine the scope of this prototype project, limiting our design development to only those systems directly related to the task of soil retrieval. Therefore, additional subsystems such as control, navigation and transportation (suspension and drive train) have been left out of this research and design report. With legitimate verification of our design’s functional capabilities, it could easily be implemented in a full-scale, prototype construction project.
History and Development of Design
The Original Design
Our original design was a complex integration of several mechanical subsystems, the basic functionality of which was based on the following progression of operational steps. These steps included sweeping topsoil onto a collection plate, transferring the soil to a conveyor, which carried the soil to a filtration system prior to being sifted into the collection bin. As evidenced by the previous list, this design was a complex aggregate of mechanized subsystems. After further analysis, we discovered we could better accommodate the project specifications by reducing the number of motorized systems, and by minimizing the number of moving parts open to the atmosphere.
The Venturi-Vac
Shortly into our design brainstorming process, we re-examined the project specifications, and investigated different technologies that could meet the original project requirements more efficiently. This led us to the development of a vacuum-based system that works by funneling atmospheric gas through a venturi (Figure 1A).
Figure 1A: The Venturi Chamber
The functional logistics of the venturi concept will be explained in more detail in the body of this report. For now, the basic idea of the venturi is that if we pump atmospheric gas (air) into the venturi tube at a given velocity, the velocity will be increased at the point where the diameter of the chamber is decreased (point A on Figure 1A). This increase in velocity will create a zone of lower pressure, inversely proportional to the change in velocity. By positioning this zone of lower pressure at the top of a tube that is open to the atmosphere (point B) at the other end, the difference in pressure causes atmospheric gas and particulate matter to be drawn up the tube. The resultant effect is similar to that of a vacuum.
Though the dimensional specifications of the venturi chamber will be detailed in later sections, it is clear that this approach allows soil to be retrieved with minimal contact between the excavation subsystem(s) and the ground. This subsequently minimizes the possibility for mechanical failure due to unforeseen obstacles, and potential breakage of those vehicle parts otherwise in contact with the ground.
Pre-Development Concept Testing
Prior to devoting precious time to the development of the venturi vacuum design alternative, we wanted to be as sure as possible that the basic concept would function effectively. We first investigated the possibility of utilizing this idea with several academic resources, including Dr. Robert Knecht, Dr. John Steele, and Dr. Ron Knoshaug of Colorado School of Mines. From these various resources, we were given direction on which physical properties of the venturi we should attempt to simulate and test, and which calculations would be pertinent for verifying the functional properties of our venturi idea. Therefore, while researching the data for our calculations, we set out to physically test our idea in the most rudimentary fashion. What follows is a summary of our experimental testing and findings, and a complete derivation and mathematical verification of the mechanical characteristics of the venturi vacuum design.
Experimental Testing
The Goal of Concept Testing
The overall goal of our experiments was to develop a device that can suction fine-grained particulate matter off the ground, so as to extrapolate that data into a design for use on the surface of Mars. From discussions with engineer Mike McGee at Lockheed-Martin, we understand much of the surface layer dirt on Mars is similar to the consistency of flour, extremely fine-grained silt [1]. Our intent was through initial physical experimentation to substantiate our test results with the mathematical analysis in which we were simultaneously engaged. With physical legitimization and mathematical verification, we could sufficiently justify the development of a venturi-based vacuum device for use on Mars.
The venturi vacuum is a device that makes use of differences in atmospheric gas (air) pressure to create a resultant vacuum, moving air and particulate matter from the zone of high pressure to that of lower pressure. Essentially, air flows from a larger diameter chamber, compresses through a smaller diameter chamber, and then decompresses as it enters into another larger diameter chamber (see Figure 1A on page 3).
Experiment Procedures
In our first experiment we attempted to create an upward flowing movement of air by blowing fine streams of air upward, near the ground, and along the inside wall of a larger uptake chamber. To accomplish this we channeled moving air from the vacuum, through a 20-ounce beverage bottle, and into two segments of 1/2-inch tubing. This tubing was then channeled into an upright segment of extendible air duct tubing. Unfortunately, this configuration yielded poor results.
In setting up a second section of experiments, we created the basic venturi chamber by connecting two 20 ounce plastic beverage bottles top-to-top with duct tape, and drilled a hole at the smaller diameter region to insert a plastic straw for the uptake tube. We then connected the vacuum output to one end of the plastic bottle venturi by way of the vacuum hose. This rudimentary implementation of our venturi concept worked surprisingly well, lifting the flour sample off the ground and subsequently blowing it out the back end of the venturi chamber.
Conclusions to Experimental Testing
We found that our basic approach to the venturi chamber should be effective for our purposes. We also determined that a dual venturi system (Figure 2A) would work as well as the single venturi chamber. However, the dual venturi idea offers additional benefits, including increased reliability in the event one venturi becomes inoperable, as well as an increased retrieval rate.
Figure 2A: Dual Venturi Chamber Test Configuration
Analysis of the Venturi Concept
Technical Information
The basic concept of a venturi is the primary driving force of the Venturi-Vac. The inner chamber area (see Figure 3A) provides a low-pressure area, which draws the soil and atmospheric gas up the extension tube (see Figure 3A). The first key to making the venturi operable is determining the terminal velocity of the particulate matter we intend to excavate, which is the required velocity to pick up a 1mm-diameter particle with the extension tube. Once the terminal velocity is determined, the initial fan velocity can be determined using Bernoulli’s equation. The Venturi-Vac is the best design because we can adjust the fan speed in order to pick up soil of a specified size. The following scientific information can be established for both Earth and the Martian environment.
Figure 3A: Cross Section of the Venturi Chamber
Terminal Velocity
Observe the free body diagram illustrated in Figure 3B. The buoyancy force, FB, is the force exerted on a floating soil particle. The buoyant force exerted on a floating soil particle is equal to the weight of the fluid displaced by the soil particle. The weight of the soil particle, W, is the gravitational force exerted on the soil particle. The drag force, FD, is the resistance force on a soil particle moving through a gas.
Figure 3B: Free Body Diagram for Soil Particles
To determine the terminal velocity range we had to establish a range for the Reynolds number (Re). The Reynolds number is directly related to the drag coefficient (Cd).
We believe the Reynolds number to be in the transition region based on two resources. The first being that when we tried solving for the velocity in the other regions, the Reynolds number bounds were not satisfied. The second is based on the advice from the technical advisor of this project, Dr. Robert Knecht.
By combining the forces in the free body diagram Figure 3B, we get the following.
FD+FB=W
Solving for V (terminal velocity) yields [2]:
Earth calculations
Following are the values we used to solve for the terminal velocity for our earth prototype. For the density of the medium, , we used the density of air at standard atmospheric pressure at 200C, 1.204 kg/m3 [3]. For the density of the soil, , we used the actual density reported from the Mars Pathfinder Mission, 1120 kg/m3 [4]. We used, 9.81 m/s2 for the gravitational constant, g. We used the specified diameter of 1mm for d. The viscosity of the medium, , is the dynamic viscosity at standard atmospheric pressure at 200C, 1.82E-05 Ns/m2.
Since we know that the Reynolds number is between 1 and 1000, we can solve for an ideal velocity. The terminal velocity thus ranges from 0.0151 m/s to 15.348 m/s. We can now solve for the Reynolds number using various velocities in this range. In turn, now we can solve for the drag coefficient, Cd, in this range. This allows us to solve for the ideal terminal velocity. Once we have solved for the terminal velocity range, we must verify that the Reynolds number is valid i.e. in the range specified. The following graph Figure 3C shows the range of calculated ideal terminal velocities vs. the corresponding Reynolds number for our earth prototype. This verifies that the velocity is within the range and is valid.
Figure 3C: Terminal Velocity for Earthen Soil
Mars calculations
Following are the values we used to solve for the terminal velocity for proof of theory on Mars. For the density of the medium, , we used the density of carbon dioxide (CO2). This specific data was not made available to us, therefore we used a scaled version of the earth’s CO2 density. The density of CO2 on earth is 1.83 kg/m3 at standard atmospheric pressure. We then multiplied this by the percentage of difference between earth’s and Mar’s atmosphere (0.6%) [5]. Hence, for the density of the medium, , we used 0.01098 kg/m3. For the density of the soil, , we used the actual density reported from the Mars Pathfinder Mission, 1120 kg/m3 [4]. We used, 3.69 m/s2 for the gravitational constant, g [5]. We used the specified diameter of 1mm for d. The viscosity of the medium, , is the dynamic viscosity as provided from Lockheed Martin, 0.062813*T0.96E-6 Ns/m2, where T is surface temperature in Kelvins [6]. The average temperature on Mars is 210 Kelvin [5]. Therefore, the viscosity of the medium we used is 1.065E-5 Ns/m2.
Since we know that the Reynolds number is between 1 and 1000, we can solve for an ideal velocity. The terminal velocity thus ranges from 0.9699 m/s to 969.94 m/s. We can now solve for the Reynolds number using various velocities in this range. In turn, now we can solve for the drag coefficient, Cd, in this range. This allows us to solve for the ideal terminal velocity. Once we have solved for the terminal velocity range, we must verify that the Reynolds number is valid i.e. in the range specified. The following graph (Figure 3D) shows the range of calculated ideal terminal velocities vs. the corresponding Reynolds number for the proof of this theory in the Martian environment. This verifies that the velocity is within the range and is valid.
Figure 3D: Terminal Velocity for Martian Soil Particles
Bernoulli Equation Analysis
Determination of Initial Velocity
The fundamental setup for our venturi-based vacuum design can be viewed in Figure 4A. We designed the main airflow source to be split into two channels. Each channel contains one of the venturi vacs (Figure 4A). Furthermore, the two separate venturi channels increase the vacuum area for the given velocity.
Figure 4A: Basic Venturi Chamber
A rudimentary explanation of our venturi design is rooted in the mathematics surrounding Bernoulli’s equation. Therefore, now that the terminal velocity is known we can apply Bernoulli’s equation to the venturi system to calculate the pressure over the extension tube, P2, and the initial velocity, V1. Specific assumptions must be made in order to apply Bernoulli’s equation [3].
Assumptions we made include:
1) the viscous effects of the air are negligible
2) the flow is a steady flow
3) the flow is incompressible (the velocity of air is less than 3/10 the Mach Number)
4) the equation is applied on a streamline.
Symbol definitions:
pressure elevation